The suitability of a polymer material for being welded by high frequency welding is determined by two criteria:
1. Thermoplasticity; i.e., the material must change from the solid to the liquid state at a temperature below its decomposition temperature, the temperature for the change of state being preferably preceded by a softening temperature range. PA1 2. Dielectric loss factor; i.e., the capacity to absorb energy from a high frequency electric alternating field. This characteristic is a constant of the material. PA1 R.sub.1, R.sub.2, and R.sub.3 are the same or different and represent a member selected from the group consisting of PA1 e represents an integer of from 1 to 10, PA1 n represents an integer of from 2 to 10, PA1 m represents an integer of from 2 to 10, PA1 o represents an integer of from 2 to 6, PA1 Z.sup..sym. is the charge number of the cation, may be equal to or less than the total number of nitrogen atoms and is preferably 1 or 2, and PA1 u denotes the number of cations (preferably one or two) that may be present depending on the charge number Z.sup..sym., such that uZ.sup..sym. =vZ.sup..crclbar. (it is preferable that uZ.sup..sym. equal one or two); PA1 X represents an oxygen or sulfur atom; PA1 A and B are the same or different and represent a member selected from the group consisting of PA1 p and q are either 0 or 1, and when A and/or B represents an oxygen or sulfur ion, the corresponding p or q must be zero; PA1 Z.sup..crclbar. is the charge number of the anion, PA1 v represents the number of anions present, being an integer of 1 to 3, (preferably 1 to 2), such that vZ.sup..crclbar. =uZ.sup..sym. ; PA1 R.sub.4 and R.sub.5 may be the same or different and represent hydrogen or a C.sub.1 to C.sub.10 alkyl or cycloalkyl. PA1 n is a number from 2 to 4, preferably 2, and PA1 Q denotes an aliphatic hydrocarbon group having 2 to 18, preferably 6 to 10 carbon atoms, a cycloaliphatic hydrocarbon group having 4 to 15, preferably 5 to 10 carbon atoms, an aromatic hydrocarbon group having 6 to 15, preferably 6 to 13, carbon atoms or an araliphatic hydrocarbon group having 8 to 15, preferably 8 to 13 carbon atoms,
Foamed materials are generally welded, not to themselves, but to textiles or foils or pressed boards. The dielectric loss factor of the composition material is, of course, strongly influenced by these other materials and the effect of these other materials frequently overrides that of the polyurethane foam. However, a high dielectric loss factor is, nevertheless, still desirable in foams, since it enables them to be welded to any materials, including those which do not themselves have a dielectric loss factor (e.g., textiles of pure polyethylene terephthalate fibers).
Among the usual homogeneously-foamed materials, PVC foam has hitherto been the only one which amply fulfills both the conditions of thermoplasticity and of dielectric loss factor. Its high cost, high weight and poor mechanical properties, however, have prevented it from being widely used. Other materials available for high frequency welding include thermoplast-modified polyester polyurethane foams.
Polyurethane foams may be rendered suitable for high frequency welding by, for example, stitching the foam with thermoplastic fibers which have a dielectric loss factor, or by spraying thermoplastic high frequency-active powder over the foam. These methods have both practical and economic disadvantages inherent in the systems, however. Stitching requires the foam to be perforated and the stitched surface cannot be covered with surface layers by flame laminating. Spraying powder over the foam means that some powder may trickle out of the product and soil the machinery or the surface of the foam. Both these methods are expensive.
Incorporation of thermoplastic powder into the stream of raw material during the foaming process is a more efficient method of rendering the polyurethane foam suitable for high frequency welding, but the quantity which may be added in this manner is limited by the resulting sharp increase in the viscosity of the polyester polyol which is already highly viscous. Moreover, uniform distribution of the solids in the foam is difficult and the unavoidable inclusion of gas also causes problems.
Considerable progress in the production of foams capable of being welded by high frequency welding was provided by the development of polyurethane solutions in polyether polyols as described in German Offenlegungsschriften 3,008,590 and 2,937,509. Such foams may be produced as completely homogeneous products and are eminently suitable for welding with any high frequency active material. Problems still occur, however, when these foams are to be welded with surface layers which have no dielectric loss factor (e.g., textiles of pure polyester fibers), since the foam alone absorbs too little energy due to its relatively low dielectric loss factor. This low absorption of energy by the foam results in a greatly prolonged welding time or very high energy requirement, both of which increases the risk of burning the material. Although the production of high frequency weldable polyether polyurethane foams according to the teaching of these references provides considerable improvements compared with standard polyether polyurethane foams, experience has indicated that there is still considerable room for improvement.
It has now surprisingly been found that the dielectric loss factor of polyurethane foams, including those foams which may be produced according to German Offenlegungsschriften Nos. 3,008,590 and 2,937,509, may be significantly increased by the addition of the ammonium salts of phosphorus-containing acids according to the invention.
As shown by the Examples, a considerably-improved capacity for high frequency welding is achieved, even with materials which are not themselves capable of being heated by high frequency. Thus, for example, this technique may readily be employed even for welding textiles which do not have a dielectric loss factor necessitating that the entire loss factor be provided by the foam. The increase in loss factor was surprisingly achieved by introducing ammonium salts of phosphorus-containing acids into the polyurethane foams.